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You can reach a point of diminishing returns with the bolt ons where the heads become the restriction, at which point further upgrading becomes a significant decision point. On my slicktop I am running gt28rs on a stock MT block with msp manis, SZ 3-2.5" tps, SZ expansion DPs, ash massives, ash inlet and IC pipes, 60mm TBs. We had SZ up here in the spring to tune the cars. The car put down 490 at 18 psi on 94 octane but is basically limited by the stock heads. We had a couple of other cars there that clearly demonstrated that headwork including oversize valves, cams, and adjustable cam gears would really open things up. That said, I am pretty happy with 490 on the street, especially with the NA gearing so I am not going to proceed.
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